1. Field of the Invention
The present invention relates to an I/Q modulator using current-mixing and a direct conversion wireless communication transmitter using the same, and in particular, to an I/Q modulator using current-mixing and a direct conversion wireless communication transmitter using the same having an improved EVM of a wireless communication system, a linearity and a power consumption wherein a D/A converted signal is converted to a current level and then subjected to a frequency modulation through this to overcome a disadvantage of the conventional wireless communication system using a direct conversion wherein a voltage level is attenuated through a signal attenuation block due to a high voltage level of the D/A converted signal to obtain a high voltage gain in a design of an I/Q modulator which requires a large current consumption.
2. Description of the Related Art
Recently, many researches are carried out to develop a low power one-chip solution for a wireless communication system. Particularly, since a RF transmitter block consumes most of the power in the wireless communication system, an optimization of the RF transmitter block should be given priority to embody the low power one-chip solution.
A generally used transmitter in a conventional wireless communication system employs a superheterodyne type. In accordance with the superheterodyne type a signal of a low frequency band including an actual information such as a voice or an image is converted into a signal of an intermediate frequency and then transmitted using a carrier wave which is a high frequency wave. The superheterodyne type is disadvantageous in that a hardware configuration is very complex and a power consumption is large.
In order to overcome the disadvantage of the conventional superheterodyne type, a direct conversion type wherein a baseband signal is directly up-converted to a carrier wave signal without using the intermediate frequency is being widely used. The direct conversion type is advantageous in that the direct conversion type has the lowest power consumption of wireless communication transmitter structure, and has a low production cost and a miniaturization is possible.
FIG. 1 is a block diagram illustrating an example of the conventional direct conversion wireless communication transmitter. As shown, the conventional direct conversion wireless communication transmitter 100 comprises a digital modulator 110, a D/A converter 120, a signal attenuator 130, a frequency-up I/Q modulator 140 (including an I-modulator 143 and a Q-modulator 146), a poly-phase filter 150, a local oscillator 160, a power amplifier 170 and an antenna 180.
The digital modulator 110 digitally modulates the baseband signal to output an I component and a Q component signals.
The D/A converter 120 converts the I component and the Q component signals which are the output signals of the digital modulator 110 to an I component and a Q component signals of an analog data.
The signal attenuator 130 attenuates a level of the I component and a Q component signals which are the output signals of the D/A converter 120. That is, the I component and the Q component signals includes a signal of high voltage level. Therefore, since there is a linearity problem of the signal when the signal of the high voltage level is directly applied to the frequency-up I/Q modulator 140, the signal is attenuated by the signal attenuator 130 prior to the application.
The local oscillator 160 generates a sinusoidal wave having a frequency of FTX. FTX is a frequency of an output signal of the direct conversion wireless communication transmitter 100.
The poly-phase filter 150 receives the sinusoidal wave generated by the local oscillator 160 to generate an output signal having a phase of 0°, 90°, 180°, 270°, for example.
The frequency-up I/Q modulator 140 multiplexes the output signals of the signal attenuator 130 and the poly-phase filter 150.
The power amplifier 170 amplifies the output signal of the frequency-up I/Q modulator 140 to have a desirable magnitude.
The antenna 180 emits the output signal of the power amplifier 170 to an atmosphere.
Although not shown, the conventional direct conversion wireless communication transmitter 100 further comprises a DC matching and analog filter. Generally, the output signal of the D/A converter 120 includes a plurality of harmonic wave signals, and a DC level of the output signal differs from that of the frequency-up I/Q modulator 140. Therefore, the DC matching and analog filter removes the harmonic wave signals and performs a DC matching to match an DC output level of the D/A converter 120 and a DC input level of the frequency-up I/Q modulator 140.
In accordance with the conventional direct conversion structure described with reference to FIG. 1, the signal is converted to a desired carrier frequency through only one frequency up-conversion by the frequency-up I/Q modulator 140. Therefore, a performance of the frequency-up I/Q modulator 140 in a transmitter using the direct conversion structure has a large effect on a characteristic of the entire modulation quality thereof such as a EVM (error vector magnitude), a linearity and power consumption.
FIG. 2 is a configuration diagram illustrating a conventional I/Q frequency up-conversion modulator.
As shown, I/Q signals VIP, VIN, VQP, VQN of the baseband which are the output signals of the signal attenuator 130 and output signals LO0, LO90, LO180, LO270 having phases of 0°, 90°, 180°, 270° which are the output signals of the frequency-up I/Q modulator 140 are applied to the I-modulator 143 and the Q-modulator 146 to generate output signals VON, VON. A load Zload is a load impedance for the output signals VON, VON.
However, as described above, a signal attenuated by the signal attenuator 130 are input to the I-modulator 143 and the Q-modulator 146 in case of the I/Q modulator for the conventional direct conversion type wireless communication transmitter. Therefore, a voltage gain should be increased since the frequency-up I/Q modulator 140 performs a frequency modulation on a signal having an attenuated voltage level. Due to this, a large current consumption is required in order to obtain a high voltage gain during a design of the frequency-up I/Q modulator 140.
Such configuration degrades the linearity which is one of most important characteristics of the frequency-up I/Q modulator 140 even when the attenuated signal is used by the signal attenuator 130. In addition, a relatively large area is required for the design of the frequency-up I/Q modulator 140 to obtain the high voltage gain.
Therefore, a method for improving the important characteristic such as the EVM, the linearity, the power consumption, the modulation quality without using the signal attenuator in a wireless communication system using the direct conversion structure is necessary.